Metamaterials and Transformation Optics
Metamaterials may be classified into two categories: LSPR-based and non-LSPR metamaterials. LSPR-based metamaterials such as negative index metamaterials rely upon LSPR phenomenon and hence the best performing plasmonic materials are noble metals followed by metal nitrides, further followed by TCOs (Tassin et al., 2012). Non-LSPR metamaterials rely upon effective medium behavior of composite materials and examples are cloaks, hyperbolic metamaterials, and epsilon-near-zero (ENZ) materials. As a general note, most of the metamaterial implementations of transformation optics fall into the category of non-LSPR metamaterials. Such metamaterials benefit a lot from alternative plasmonic materials because of their smaller Im{εm}.

Hoffman et al. showed negative refraction at an operating wavelength of 8 µm in a so-called hyperbolic metamaterial consisting of planar, alternating layers (a superlattice) of heavily doped (1–4×1018 cm−3) In0.53Ga0.47As and undoped Al0.48In0.52As deposited by MBE (Hoffman et al., 2007). This metamaterial device was shown to have a performance figure-of-merit of about 20, the highest reported so far. Yet another demonstration is the epsilon-near-zero (ENZ) properties of the InAsSb material when doped heavily. Adams et al. show that heavy doping (1–2×1019 cm−3) of InAsSb causes the real permittivity to cross zero and turn metal-like in the MIR range (Adams et al., 2011). At the zero cross-over of real permittivity (occurring at about 8 µm wavelength), the material behaves as an ENZ material and exhibits special properties such as photon funneling.